1. Field of the invention
This invention relates to a seam welder and, more particularly, to a seam welder, in which upper and lower roller electrodes are disposed in a face-to-face relation, wire electrodes are inserted between said upper and lower roller electrodes, a metal plate or covered metal plate is guided continuously at a constant welding speed between said wire electrodes for effecting welding through heating by passing current, and a pulse voltage with the polarity thereof varying alternately is supplied between said upper and lower roller electrodes to energize said base material, thus varying the frequency of the pulse voltage according to variations of the welding speed to hold the welding current at a constant value.
2. Prior art
Heretofore, with a seam welding known as one of resistance welding techniques a welding section excellent in the liquid tightness can be obtained, so that it is used for the welding of liquid transportation pipes, fuel tanks, drum cans, food cans, etc. Among the seam welding the most general one is the lap seam welding. According to this lap seam welding, a base plate, such as a steel plate or like, is formed into a cylindrical body, and then the cylindrical body is moved between upper and lower roller electrodes opposed to each other, by which a pressure is applied to an overlap section of the cylindrical body, the overlap section being constituted by the edges of the base material lapped over. Between the upper and lower roller electrodes, an intermittent current flow through the overlap section, on which such pressure is applied, generates a joule heat and a subsequent partial fusion, so as to effect seam welding. In the seam welding method, a large amount of current can flow in a short period of time to increase the welding speed. Therefore, it is suited for mass production. Also, in the seam welding method, unlike spot welding, nuggets is continuously formed and partly, overlapped, so that a continuous seam welding section is formed. For this reason, in the seam welding, a welding joint can be obtained, which is excellent in the air tightness and water tightness. From this standpoint, this welding process is said to be suited for food cans and drink cans.
However, the drink cans and food cans are required to have corrosion resistance against the content. the cold-rolled steel plate is not directly used as the base material, but use is made of a covered steel plate, e.g., tin-plated steel plate. However, when the cylindrical body formed by such covered steel plate is subjected to seam welding by direct pressure application by the upper and lower roller electrodes, the covering metal, such tin or the like, existing on the surface of the covered steel plate, like transferred to the outer surface of the roller electrode, i.e., contact surface, forming contamination thereof. Unless such contaminant is removed off the contact surface of the roller electrodes, satisfactory welding can not be obtained.
From this standpoint, there has been proposed a seam welder as disclosed in Japanese Patent Publication 25213/1944. In this seam welder, as shown in FIG. 1, wire electrodes 3, i.e., copper wires, are provided on the outer periphery of the upper and lower roller electrodes 1 and 2, and an overlap section 4a of the cylindrical body, which is formed by the base material 4 is seam welded continuously between the wire electrodes 3. In this case, the upper and lower roller electrodes 1 and 2 are continuously rotated, and the outer periphery of the upper and lower roller electrodes 1 and 2 is provided with a guide groove. The wire electrodes 3 are inserted in these guide grooves and are fed together with the base material 4 with the rotation of the upper and lower roller electrodes 1 and 2. A large amount of current flows through the overlap section 4a between the wire electrodes 3 intermittently by the upper and lower rollers 1 and 2, so that the overlap section 4a is subjected to a partial fusion and seam welding thereof.
In this welder, a welding current Iw is fed intermittently between a pair of electrode rollers 1 and 2 at a predetermined frequency as shown by solid line in FIG. 2(a). Only at the time when the current flows through the overlap section, the overlap section is thermally fused, and nuggets 9, which are continuous to one another as shown in FIG. 2(b), are formed resulting in seam welding of the overlap section.
More specifically, in FIG. 1, a commercial three-phase AC power source drives a generator 5, from the output side of which a constant single-phase AC voltage is generated at a constant frequency. This single-phase-phase AC voltage is phase controlled by a rectifier 6 provided with a pair of thyristors 6a and 6b connected in opposite polarities so as to control the control voltage EC having a waveform as shown by dashed line in FIG. 2(a). The control voltage Ec is supplied to the primary side of the transformer 8 through a resonant capacitor 7. On the secondary side of the transformer 8 the voltage is increased, a welding current Iw flows between the pair roller electrodes 1 and 2 at a constant frequency as shown by solid line in FIG. 2(a). Correspondingly, nuggets 9 are formed in the overlap section of the cylindrical body formed by the base material 4, thereby obtaining a seam welding portion with the nuggets 9 in an aligned sequence, in which overlapping portion is formed between adjoining nuggets 9.
In this welder, however, the period or frequency of the single-phase AC voltage provided by the generator 5 is constant at all time, so that only a welding current Iw having a constant frequency can flow between the pair roller electrodes 1 and 2.
Therefore, if the welding is done at a welding speed in a range suited to the frequency of the welding current, the individual nuggets 9 partly overlap one another as shown in FIG. 2(b). When the welding speed gets out of the suitable range noted above, the nuggets no longer partly overlap, so that it is impossible to obtain a weld joint which is superior in the air tightness and water tightness. In other words, the proper welding speed is determined by the frequency of the single-phase AC voltage generated by the generator 5. In the prior art welder, however, a single-phase AC voltage with the frequency thereof varying continuously over a wide range can not be generated over a wide range. Therefore, an optimum welder is necessary according to the welding speed. This constitutes a large problem.
Further, in the case of FIG. 1 the output voltage of the generator 5 driven by a commercial three-phase AC supplied from a substation through a motor or the like is varied according to variations of the commercial three-phase AC current, and the welding condition is varied according to variations.
More particularly, the sudden fall of the commercial three-phase AC voltage is caused due to a phenomenon, such as the thunder, heavy rain, etc. With this rainfall the output voltage of the generator 5 falls, and the frequency of the generator 5 is varied with variations of the rotation of the motor or the like for driving the generator 5. For this reason, in seamwelding portion is generated regions where the welding current is insufficient, during welding process. As a result, by such insufficient current region, a partial formation of nugget with a reduced areais made during continuous formation of nuggets. Also, an existence of the area-reduced nugget makes a discontinuity between nuggets in sequence, because the area-reduced nugget can not be overlapped with adjoining nugget. Therefore, as to the resultant welding beads, mechanical strength is deteriorated.